vboot_reference/futility: Split load_firmware_image() fn
vboot_reference/futility: Split load_firmware_image() fn

This is in prep for removing the need for temp files.

V.2:
 Move validation into parse_firmware_image()

BUG=b:203715651
BRANCH=none
TEST=cros deploy to nocturne and ran:
 `/usr/sbin/chromeos-firmware --mode=recovery`.

Signed-off-by: Edward O'Callaghan <5ed2e9bf3d98d5f12a86c198971e1dafd1bbd008@google.com>
Change-Id: Id61fcb0f53546a78085e0a367c21780c5885bc51
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3244679
Commit-Queue: Edward O'Callaghan <5ed2e9bf3d98d5f12a86c198971e1dafd1bbd008@chromium.org>
Commit-Queue: Sam McNally <fa45cad00d68e10cba722214c28ca2497355b0cc@chromium.org>
Tested-by: Edward O'Callaghan <5ed2e9bf3d98d5f12a86c198971e1dafd1bbd008@chromium.org>
Auto-Submit: Edward O'Callaghan <5ed2e9bf3d98d5f12a86c198971e1dafd1bbd008@chromium.org>
Reviewed-by: Sam McNally <fa45cad00d68e10cba722214c28ca2497355b0cc@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot